It's time to replace the carb the PO put on the car and I've been debating between a Holley dbl pumper or vacumn operated secondary Avenger. (the current carb is 30 yr old rebuilt, ridiculious 780 cfm!!) I'd like to stick with Holley since I have had them for 10 years and I understand them. I typically almost never wind my baby out past 5000rpm and I am looking for more low end torque. I don't drag race but I do have a heavy foot. So, the questions are:

- How many CFM?
- Which Holley model?
- Do I go with the mechanical secondaries or spend time tuning the vacumn operated secondaries?
- How much fuel economy is lost between tuned vacumn secondaries and mechanically operated secondaries?

The vital statistics are;
- 350 cu in with 11:1 compression running on 97-99 octane
- Engle cam - 491 lift, 274 duration (238 @ .050)
- dual plane intake with med rise- very similiar to '70LT1
- tube style headers feeding 2.5" duals & turbo II's
- M20 wide ratio 4 speed with 336 gears

I enjoy the performance I get from my small block and I would tend to place my overall emphasis on performance. However, high octane fuel isn't cheap and I'm not willing to suffer a substantial drop in fuel economy just to gain a slight edge.

Once again, thank you (in advance) for your input!

For a streetcar, I always recommend vacuum secondary. I have had good luck with my #4175 from Summit Racing. $300 All I changed on it was the discharge nozzle and accelerator cam.

Hello Rick, I haave just gone through this again....... notice I said again. I did it 2 years ago, and just again last week. I had a Performer intake and purchased a new Holley 4175 #80555 carb, 650 single feed, with vacuum secondaries with electric choke. Well a guy that works for me said his brother wanted to get rid of some of his old small block things, he had a set of 69cc heads that were hogged out, new valves and springs, also a preformer RPM intake and matching cam and lifters, all I needed was a aset of rockers he kept his rollers. So I ordered a new set of stainless rollers, and he sold me all the goods for 200, and that included him helping me install and tune. Well we get everthing pretty much together, and hes looking at my carb, its a square body , the RPM intake is a older type and only drilled for a Holley. And they don't make a adapter that goes that way,we looked, so its either get a new intake or carb. I should have went with the intake it would have been cheaper, but it was installed and besides it was port matched to the heads, so I reluctlantly went a bought a new carb. I got a 600 single feed with vacuum secondaries and electric choke. We did the formula and if your getting over 6500 rpms and this and that, hell if I see 6500 its a mistake!, so I went with the 600. So after all that if you want a 650, it is a square body like the quadrajet, its less than 2 years old, probally less than 5000 miles, I'll sell it for lets say 150.00 and I'll pay shipping. A 300.00 carb, worked great and I would still have it but I made the other choice. Also I have another 600 cfm holley square bore here, the one with metering rods, not sure of the number on that on though, also a year old, but that one has no choke assembly at all, got removed if you know what I mean........

I guess alittle more information on your carb selection, they make either a single or dual pumper. The single is the vacuum secondarie so 1 fuel pump and usually single feed with a transfer tube feeding the secondarie bowl. The double pumper usually had 2 fuel inlets one for each bowl, and a pump for each primary and secondarie. The vacuum secondarie model for street, and racing, the double pumper is for racing no fuel economy hard core enthuiasts.

Thats about all I have, hope it helps in your selection, let me know if your intrested in the carb or email me if you have any questions.

Thaks Dave
Mr69vett

Thanks Dave for your input. I too have an intake (and a very old one at that) that fits only a Holley so the spread bore carbs won't work for me either. Thank you for your offer though.

Also, what you refer to as single or dual pumper is the same as what I mean by Dbl pumper or vacumn secondary. The double pumpers are supposed to be fantastic with a manual transmission, what I'm wondering though is just how bad is the fuel economy. I've been told that if I want more punch with a vacumn secondary I can just change the diaphram spring to a lighter one so that the secondaries come in much sooner.

By the way, I wouldn't worry about that 600cfm being too small! It always amazes me how many guys grossly overcarb their engines. Plus, from what I know you should have a better low end with the smaller carb. Sounds like a phenomonal deal on your heads and intake. Its worth the $200 alone just for the install and tuning help!!

Iyour really looking for the right carb, email or call holley tech service. I will however put in my two cents. Its a stick car so Double Pump it! The double pump will be crisper down low when leaving the light and there is less chance of stalling the engine on a stick car with double pump then with a vacuum secondary.

Engine Compression. Your stuck with the high octain until you change the pistons or the heads. You might be able to drop the compression with a thicker head gasket. Several gasket companies are making .051 gaskets and if your running a copper shim or a .038 you could possibly drop that number a half point with just a gasket.

Gas mileage. Its all in the foot and partly in the tune up. If you can get it as close to 12:1 ratio you will save some gas, but what you need to under stand is High compression = more fuel consuption then an engine runing 8:1 even at idle. The more HP you make the more fuel you need to make that HP and compression = HP = more fuel. Look at winston cup engines. The restrictor plate engines run a 390cfm carb, but they still requires almost the same amount of fuel to get it to the finish line. If you want to stretch fuel its how you drive it, the cup guys even know that and if you listen to the crew chiefs they will tell them to conserve fuel by getting in and out of the throttle later in and out of the corners.

Considering your engine and no more RPM's then you will be turning it, my suggestion is you go with a 650 double pump carb. Not an edelbrock 650, their 650 only flows 500 cfm. Their 650 CFM rating is a dry flow number, not wet.

After Shark said: Gas mileage. Its all in the foot and partly in the tune up. If you can get it as close to 12:1 ratio you will save some gas, but what you need to under stand is High compression = more fuel consuption then an engine runing 8:1 even at idle. The more HP you make the more fuel you need to make that HP and compression = HP = more fuel.

I'm not following....feel free to correct any mis-conseptions I may have.
Fuel is drawn into the engine on the intake stroke . Compression is increased by reducing the amount of area in the combustion chamber, ie. smaller chambers and/or raising the top surface of the piston.
By swapping in flat top pistons and 64cc heads on my L48, but retaining the 5.7 stroke, I am not changing the volume of fuel/air coming into the engine, only the amount of pressure applied to that fuel/air mixture during compression. Granted you may need a slightly richer mixture, but generally, and I said generally, most engines perform best with a mixture between 12.7 and 13.1 ratios.
I think you may also be giving too little credit to propper tuning.
I am looking at an article right now:
1970 Vette with a 350. They added an Accel ignition system w/vacuum advance and an Accel air filter. From there they began tuning: air/fuel ratio, spark advance, plug gap and so on. Results, 44hp and 42lb-ft of torque gained w/throttle response, idle quality, and fuel mileage improving.

|UPDATED|6/13/2003 9:07:41 PM|/UPDATED|

There is a formula on the Holley web site to determine the correct carb size per given engine.
Displancment X Red Line / 3456 = CFM
350 X 6500 / 3456 = 658 cfm.
They also say that a stick shift should have mechanical secondares ( double pumper ) and an auto trans should have vaccuum. ( no clue as to why ).
If I use the same formula for my 427, it comes out to a 654 cfm carb ( my red line is 5300 ). Speed shops and garage mechanics keep telling me that it is not enough of a carb and I should go with at least a 750. ( I will eliminate a very long story at this point but when I got the car back it had a new 780 ( or 770 ) street avenger with vaccuum secondarys ( as per local advice ) and the car lost all it's punch off the line and bogs terribly. I'm not happy. Time to rebuild the 650 spread bore and either put the Holley on e-bay or try it on my Chevelle.
Anyhow, that is my experiance.

Is it a bog or a Hesitation? A bog usually means the engine is getting to much air or fuel to accelerate properly. Before rebuilding I would call Holley tech and work out the problem. Usually a stiffer spring on the vacuum secondary side will let the secondaries open slower and elinate the bog. If its hesitating then you need a bigger pump squirter.

All the magazine did was improve the efficiency of the engine. I don't have the article so I can only speculate on a couple things. Air filter - cotton and foam flow better then paper. If they improved the air flow and didn't change the mixture during that step then its possible to improve the fuel air ratio. You also have to realize that the engine doesn't have to work as hard to accept the air. No different the us trying to breath through a straw.

Ignition - Here is one area that can really improve an engine and there are lots of variances that come into play. Spark out put is a big one and restrikes is another. Most after market ignitions will fire the a plug with more energy and multisparks in timing curve for each cylinder. What happens with that energy is it allows the fuel/air mixture to burn harder and more efficently.

Spark plug gap changes are very common for aftermarket ignitions in order to get best performance out of it because it needs a wider gap to cross to carry the energy as well as for the multiple restrikes.

Compression - You are correct, but what you also have to remember is the engine is just a big pump! All they did in the article was improve the effiency of the pump. When you squeeze the air/fuel mixture on the intake, the spark is what ignites the mixture and forces the piston down. What they did was by adding the igntion and making it easier for the air/fuel to enter the engine and making the fuel/air mix better allowed the it to make a bigger boom in the cylinder sending it down with more energy. If you do that in every cylinder then you have the possibility to make more HP.

Now if they upped the compression from this point they would probably need to add more fuel, to keep from causing a lean detonation. The tighter you squeeze the mixture the more heat is created. The way you cool it is fuel or a higher octane that has a higher volotility so it burns quicker.

I'm I confusing the matter or am I making any sense.

I vote for mechanical secondaries, but not the 4-corner idle type - they are just controlled leaks. You have all the right parts for a DP - std trans, rear end gears, compression and a good size cam.

As far as using a DP with an auto trans, the problem is when you floor the throttle the secondary shot dumps before the trans has time to downshift. With a 4-speed you floor it after the shift, so the extra gas has somewhere to go. Someone posted the other day about adding another auto trans kickdown switch that closed at 1/2 throttle and it made his (mech sec) car a lot faster.

Joe